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Abstract

Among the objectives for large-scale quantum computation is the quantum interconnect: a device that
uses photons to interface qubits that otherwise could not interact. However, the current approaches
require photons indistinguishable in frequency—a major challenge for systems experiencing different
local environments or of different physical compositions altogether. Here, we develop an entirely
new platform that actually exploits such frequency mismatch for processing quantum information.
Labeled “spectral linear optical quantum computation” (spectral LOQC), our protocol
offers favorable linear scaling of optical resources and enjoys an unprecedented degree of
parallelism, as an arbitrary N-qubit quantum gate may be performed in parallel on multiple
N-qubit sets in the same linear optical device. Not only does
spectral LOQC offer new potential for optical interconnects, but it also brings the ubiquitous
technology of high-speed fiber optics to bear on photonic quantum information, making
wavelength-configurable and robust optical quantum systems within reach.

This electronic bandwidth is significantly smaller than the optical passband widths of Fig. 3, due to the nonlinear relationship between the phase in ϕ(t) and the coefficients ck in Eq. (6), as well as the fact that cascaded EOMs excite more optical modes than possible by their bandwidths individually. See Supplement 1 for the specific microwave spectra.

Other (11)

This electronic bandwidth is significantly smaller than the optical passband widths of Fig. 3, due to the nonlinear relationship between the phase in ϕ(t) and the coefficients ck in Eq. (6), as well as the fact that cascaded EOMs excite more optical modes than possible by their bandwidths individually. See Supplement 1 for the specific microwave spectra.

Figures (4)

Building blocks for spectral LOQC. (a) Dual-rail qubit encoding. A single photon corresponds to
|0⟩L or |1⟩L depending on which one of two modes it occupies.
(b) Fourier-transform pulse shaper. This applies arbitrary phases to each spectral mode,
physically, by separating and recombining frequency components (left), and conceptually, as a
multimode element operating on all rails individually (right). (c) Electro-optic phase
modulator. This device (left) applies an arbitrary temporal phase periodic at the inverse mode
spacing. In rail form, the modulator acts as mode mixer that can move photons across frequency
states (right). The labels A0 and A1 mark the zero and one modes for a representative qubit.

Schematics of spectral LOQC gates. Each rail represents a distinct frequency mode, in increasing
value from top to bottom. The logical zero and one modes for the first qubit are labeled
A0 and A1; those for the second qubit are B0 and B1. The labels PS and EOM denote pulse shaper and electro-optic
phase modulator, respectively. (a) Hadamard gate. With two pulse shapers and two EOMs, this
operation succeeds with probability 1, requiring no ancillas. (b) CZ gate. Two ancilla photons
are loaded in the modes adjacent to A1 and B1, and all photons propagate through a series of
R pulse shaper/EOM pairs. The spectrally resolved detection
pattern shown here then heralds successful completion of the gate.

Bandwidth scaling. These plots show the performance of the spectral Hadamard and CZ gates as a
function of optical bandwidth, in terms of fidelity F and success probability P. (a) Hadamard gate: fidelity (top) and success probability
(bottom). (b) CZ gate: fidelity (top) and success probability (bottom). All three configurations
from Table 1 are tested: R=2 (green), R=3 (red), and R=4 (blue). The black line in the bottom plot marks the value
0.0741—the best linear optical CZ gate known to date.